There are two different ways to make a major hardware change with WinXP and not have to reload the OS and all your programs. The first way is to change out the hardware and then boot the system with the XP installation disk. Skip the first section that list recovery and repair options and proceed to the "Select a partition to install Windows to", choose the existing installation, you will be asked if you want to repair the installation, answer yes. Windows will re-install itself, pick up any changed hardware but leave all your programs and settings intact.
The second and more difficult method is to install the Window backup utility from the XP CD and run a full system backup on the old system saving the backup file on preferably a separate HD but definately on another partition.
Now change the hardware and boot from the XP CD. Quick format the existing OS partition, then install the OS as you normaly do. When the new system comes up, install the Backup utility and run the program choosing the backup file you made previously. Windows will restore everything from the "old system except the old hardware of course.
The second method is safer as the first method can hang up mid way through the install putting the process in a unresolvable loop forcing you to reformat and loose everything. While both methods do a decent job of restoring every setting, your bookmark order might be scrambled and any Windows security updates or other core updates will need to be reinstalled.
Both methods will also transfer all the unwanted registry entries and leftover garbage from old program and driver installs, one good reason to format and reinstall everything from scratch.